In fact that's how most shots were done in WW2 (US and germans, OŽKane used it normally): You preset a solution (Bearing, AOB, Speed) for the moment the target arrives at a certain point, and then simply wait until it reaches the crosshair you have previously fixed aiming at that point. Note that he didn't activate the position keeper, which would have ruined the pre-set shot.
Main advantage of that method is that, if all data were correct, you can target any specific part of the ship you wish. F.e. if you shoot when the bow crosses the point of aim, you will hit the bow, if you shoot when the engines go through you will hit them, and so on.
In real US subs you could do that while having the PK started to check if the target does what expected. In SH4 you can't have at the time the PK activated and a solution present for firing at a specific bearing
Thus you must deactivate it.